Enable needs to be pulled either High (5V) or Low(5V) to enable the device.If DIR is low(0V) the motor will rotate 1 direction, if DIR is High(5V) it will rotate the other direction.Every time STEP goes from either High(5V) to Low (0), or Low(0v) to High(5v) (It will be one combination or the other, but not both. the motor will move 1 step.

The reason I am a little vague here is that I don't know wheter the Enable is active High or active low. Active Low is generally a safe bet for Enable and STEP. If you have the controller connected to the motor and power it up, when the motor is anabled it will be much harder to turn by hand as the coils are then energized.

If the enable is active low then I would assume that step is also an active low signal. To make 1 step you take the STEP pin to 0v and then return it to 5v. Every time that cycle is repeated the motor will take 1 step.